1. Field of the Invention
The invention relates to an electromagnetic engagement apparatus that engages by producing a thrust based on attractive force by electromagnetism and on differential rotation.
2. Description of the Related Art
In a power transmission system of a vehicle, a clutch apparatus that permits and prevents the transfer of rotation and power of an engine is provided between the engine and a manual transmission. There are various types of clutch apparatuses; for example, a dog clutch, a friction clutch, a fluid clutch, an electromagnetic clutch, etc. are known. Among these, the electromagnetic clutch apparatus permits and prevents the power transfer by using electromagnetic force. Electromagnetic clutch apparatuses can be divided into a pressure engagement type, a dog type, a gap type that uses iron powder as a medium, etc., according to the structure that transmits torque.
For example, Japanese Utility Model Application Publication No. 5-62747 (JP 5-62747 U) describes an electromagnetic brake in which divided armatures that are magnetically attracted by electromagnetic force are employed and each armature is equipped with a braking spring. JP 5-62747 U also describes that the spring force of the braking spring that provides spring force to the first armature to be magnetically attracted or drawn is smaller than the spring force of any other braking springs.
Besides, Japanese Patent Application Publication No. 11-51089 (JP 11-51089 A) describes an electromagnetic clutch that has an armature stack formed by two divided layers, and also has a pressing spring that exerts spring force on the armature, and a buffer member disposed between the layers of the armature stack.
However, in the structures described in JP 5-72747 U and JP 11-51089 A, two armatures that are radially outer and inner armatures are provided, and return springs are provided individually for each armature, and each armature is attracted by electromagnetic force. Therefore, in the construction described in JP 5-62747 U, the electromagnetic force for releasing the clutch needs to be greater than or equal to the elastic force of the aforementioned return springs, and large current must be passed through in order to release the clutch; thus, there is room for betterment in terms of improving energy efficiency.